The present invention relates to switching circuits, such as converters, switching supplies, choppers, undulators, etc. These circuits often comprise rectifier diodes in series with inductive loads and it is necessary to use fast diodes, i.e. diodes which block rapidly during the inversion of the voltage at their terminals.
However, in view of the stray or parasitic inductances and capacitances of the circuit in which these diodes are located, at the time of blocking, current and voltage oscillations often occur. These oscillations lead to disturbances in the circuit, such as, e.g. an abnormal reswitching of the transistors in conduction or interfering signals prejudicing the control logic of the circuit. They also cause interference on the output voltage of the circuits (supply systems, undulators, etc.) and radio interference.
FIG. 1 shows a typical prior art rectifier circuit positioned at the terminals of the secondary winding ES of the output transformer TS of a switching-mode supply (primary winding EP being in series with a switch T.sub.p operated at the switching frequency). This circuit comprises a fast rectifier diode D1 in series between the secondary winding ES and the inductive load L,R and another diode D2 in parallel with the load only. In FIG. 1, a stray inductance (cabling inductance) and stray capacitance (of diode D1) are shown in dotted line form.
FIG. 2 shows the waveshapes of current I1 in the rectifier diode D1 and the voltage V1 at its terminals at the time of switching, i.e., at the time of the transition from a high direct current Id to a zero current and a low direct voltage Vd to a high negative reverse voltage Vi. This transition has high amplitude oscillations, which need to be heavily damped without leading to excessive losses and without using a large number of components.
What generally has been done up to now for damping is to connect a series network RC in parallel with diode D1, as represented in FIG. 3. However, this leads to a number of disadvantages. The cabling parasitic inductance of the circuit produces a series RLC network, which normally is insufficiently damped, particularly for high frequency oscillations, because it is impractical to increase the value of capacitor C (for increasing damping) without leading to significant losses, (because capacitor C is charged and discharged across resistor R during each half-cycle). Moreover, an increase in the capacitance C, apart from the significant losses produced, reduces the rectifying effect. The added components, i.e., the resistor and capacitor, are relatively large and require a certain connection conductor length, which makes it impossible to reduce the cabling inductance. Finally, the resistor and capacitor cannot readily be integrated onto a semiconductor chip with diodes D1 and D2.